1. Technical Field
This invention relates to a toner system printing device which performs toners such as a printer or copier, and more particularly to a high voltage generating circuit for a high voltage required for toner system printing.
2. Related Art
In a toner-system printing apparatus such as a laser-printer or copier, toners deposited on a photosensitive drum are duplicated on a printing sheet of paper using a duplicating roller. For this purpose, a high voltage is applied to the duplicating roller. This high voltage is required to be variable in a wide range of xe2x88x921300 V to +2700 V.
FIG. 4 shows a prior art high voltage generating apparatus satisfying the above requirement. This prior art apparatus includes high voltage generating circuits 81 and 82. The high voltage generating circuit 81 serves to vary a secondary output voltage continuously within a range of 700 V to 4700 V according to the duty ratio of a PWM signal 84. The high voltage generating circuit 82 serves to generate a stabilized voltage of about xe2x88x922000 V. The secondary output of the high voltage generating circuit 82 and the secondary output of the high voltage generating circuit 81 are connected in series. Therefore, the voltage at a high voltage output 83 varies continuously within a range of xe2x88x921300 V to +2700 V according to the duty ratio of the PWM signal 84.
The, above high voltage circuit will be explained below in detail. A transistor for oscillation (hereinafter simply referred to xe2x80x9ctransistorxe2x80x9d) Q81 with a collector connected to a primary coil L81 and a base connected to an auxiliary coil L83 carries out self-exited oscillation in a non-saturated range. A voltage control circuit 86 controls the oscillating amplitude of the transistor Q81 in such a manner that an output created on the basis of a signal 85 representative of a voltage on the secondary output is sent to the auxiliary coil L83. Therefore, the voltage of the secondary output is stabilized to a voltage represented by the duty ratio of the PWM signal 84 (this technique is referred to as a first prior art).
One of prior arts related to a switching power source is disclosed in the Unexamined Japanese Patent Application Publication No. Hei11-206116. In this technique, in order to limit a voltage from DC electric power to a voltage (second voltage value) or lower which is lower than a prescribed output voltage (first voltage value), a constant voltage control means is provided for feedback-controlling a switching means on the basis of an induced voltage from an auxiliary winding. The constant voltage control means takes out the induced voltage at the same timing as the timing when the output is taken out from the secondary winding. Therefore, an increase in the voltage from the DC electric power is detected as an increase in the induced voltage from the auxiliary winding. When the induced voltage is just about to exceed a prescribed value, the control of suppressing the increase in the voltage from the DC electric power is performed. Therefore, when the load is abruptly changed from its heavy state into an opened state, an increase in the voltage from the DC electric power is limited to the second voltage value (this technique is referred to the second prior art).
[Problems that the Invention is to Solve]
However, use of the first prior art presents the following problem. The transistor Q81 performs the self-excited oscillation within a non-saturated range. Therefore, the oscillating waveform at the collector at the transistor Q81 is a sine wave, but contains distortion. The ratio of distortion varies as hfe of the transistor Q81 varies. The ratio of distortion also varies when the voltage value at the secondary output and the load current value vary. In short, the ratio of distortion varies for different apparatus. Even with the same apparatus, it varies according its state of use. FIG. 2 shows an oscillating waveform containing the above distortion. Where the oscillating waveform contains such distortion, the voltage of the coil output rectified and smoothed over period t1 is higher than that rectified and smoothed over period t2.
On the other hand, the secondary output of the high voltage generating circuit 81 is the double-voltage rectified output from the secondary coil. Therefore, the voltage of the secondary output is a sum of the voltage rectified over the period tl and the voltage rectified over the period t2. However, the signal 85 representative of the voltage of the secondary output is either one of the voltage rectified over the period t1 and the voltage rectified over the period t2. Therefore, where the voltage rectified over the period t1 is used as the signal 85, the voltage represented by the signal 85 is higher than an actual secondary output voltage. Thus, the voltage of the secondary output is controlled to be lower than the voltage directed by the PWM signal 84. On the other hand, the voltage rectified over the period t2 is used as the signal 85, an opposite state occurs. Namely, the voltage of the secondary output is controlled to be lower than the directed voltage. As a result, in either case where the voltage is rectified over the period t1 or t2, the accuracy of the voltage of the secondary output is deteriorated, thereby attenuating the printing quality of the portion printed using toners.
In order to obviate such inconvenience, it has been proposed to correct the deviation of the voltage represented by the signal 85 from an actual voltage of the secondary output with the aid of a variable resistor 87, thereby preventing deterioration in the voltage accuracy. However, as described above, the ratio of distortion in the oscillating waveform also varies according the state of use. Hence, the quantity of deviation also varies according to the state of use. This presents a problem that the correction by the variable resistor 87 does not always result in suitable correction. In addition, this also presents a problem that the step of adjusting the variable resistor 87 is required.
Meanwhile, in the second prior art, the element on the primary output performs a switching operation. Therefore, in order to take out the voltage corresponding to the voltage of the secondary output from the auxiliary winding, the output must be taken out from the auxiliary winding at the same timing as the output is taken out from the secondary output. Namely, this technique is defined by the theory of switching, but is not adopted for the purpose of increasing the detecting accuracy. Therefore, in a configuration in which since the transistor performs self-excited oscillation within the non-saturated range, the oscillating waveform can be regarded as a sine wave and the voltage corresponding to the secondary output can be taken out when either half-wave of the output sent from the coil is rectified, it is difficult to apply the second prior art to the purpose of increasing the voltage accuracy at the secondary output.
This invention has been accomplished in order to solve the problems described above. An object of this invention is to provide a high voltage generating apparatus for use in a toner-system printing device capable of improving a voltage accuracy at a secondary output in a configuration in which a transistor performs self-excited oscillation within a non-saturated range, thereby increasing printing quality. In this case, the voltage accuracy at the secondary output can be improved in such a manner that, as a voltage detected signal representative of a voltage of the secondary output, a signal is used which is obtained when the output from a voltage detecting coil is rectified and smoothed at the same timing as the output from a secondary coil is rectified and smoothed by a rectifying/smoothing circuit, thereby removing a bad influence from distortion of self-excited oscillation on the voltage accuracy at the secondary output.
In addition to the above object, another object of this invention is to provide a high voltage generating apparatus for use in a toner-system printing device having a configuration in which a transistor performs self-excited oscillation within a non-saturated range and a double-voltage rectifying circuit is used as the secondary output, thereby improving printing quality. In this case, the voltage accuracy at the secondary output can be improved in such a manner that, as a voltage detected signal representative of a voltage of the secondary output, a signal is used when the output from a voltage detecting coil is double-voltage rectified or full-wave rectified, thereby removing a bad influence from distortion of self-excited oscillation on the voltage accuracy at the secondary output.
In addition to the above objects, still another object of this invention is to provide a high voltage generating apparatus for use in a toner-system printing device, capable of preventing reduction in the voltage accuracy at a secondary output due to a change in a forward voltage of a rectifying element in such a manner that the ratio of the forward voltage of the rectifying element for rectifying an output from a secondary coil to that of the rectifying element for rectifying an output from a voltage detecting coil and the ratio of the number of turns of the secondary coil to that of the voltage detecting coil are approximate values to each other.
In addition to the above objects, a further object of this invention is to provide a high voltage generating apparatus for use in a toner-system printing device, capable of improving the accuracy of an output voltage in such a manner that the ratio of the capacitance of a smoothing capacitor in a rectifying/smoothing circuit at the secondary output to an output current and the ratio of the capacitance of a smoothing capacitor in a rectifying/smoothing circuit of the detecting signal creating circuit to an output voltage are approximate values to each other.
[Means for Solving the Problems]
In order to solve the above problem, the high voltage generating apparatus for use in a toner system printing device according to this invention is applied to a high voltage generating apparatus for use in a toner system printing device comprising: a transformer wound by a primary coil, a secondary coil and an auxiliary coil; a transistor for oscillation with a collector connected to the primary coil and a base connected to the one terminal of the auxiliary coil, the transistor performing self-excited oscillation within a non-saturated range; a rectifying/smoothing circuit for rectifying/smoothing an output from the secondary coil to create an secondary output; a detecting signal creating circuit for rectifying/smoothing an output from a voltage detecting coil to create a voltage detecting signal indicative of a voltage of the secondary output; and a voltage control circuit for supplying a DC voltage control signal created on the basis of the voltage detecting signal to the other terminal of the auxiliary coil, wherein the voltage control circuit controls the oscillating amplitude of the transistor using the DC voltage control signal to stabilize the voltage of the secondary output, and the secondary output is led to a high voltage applying portion of a toner system printing section, and is designed so that the detecting signal creating circuit rectifies the output from the voltage detecting coil at the same timing as the rectifying/smoothing circuit rectifies the output from the secondary coil.
Specifically, where the output from the secondary coil and the output from the voltage detecting coil are rectified at the same timing, even when the degree of distortion varies in the self-excited oscillation, the ratio of the rectified/smoothed voltage (voltage of the secondary output) of the output from the secondary coil to the rectified/smoothed voltage (voltage of the voltage detecting signal) of the output from the voltage detecting coil does not vary. For this reason, the voltage accuracy at the secondary output is improved.
In addition to the above configuration, this invention is applied to a high voltage generating apparatus for use in a toner system printing device in which the rectifying/smoothing circuit is a double-voltage rectifying circuit, and is designed that the detecting signal creating circuit is a double-voltage rectifying circuit or a full-wave rectifying circuit.
Specifically, the output from the detecting signal creating circuit is a full-wave rectified output of the voltage detecting coil. Therefore, the output from the detecting signal creating circuit is an output rectified at the same timing as the rectifying/smoothing circuit performs rectification. Thus, even when the degree of distortion varies in the self-excited oscillation, the ratio of the rectified/smoothed voltage (voltage of the secondary output) of the output from the secondary coil to the rectified/smoothed voltage (voltage of the voltage detecting signal) of the output from the voltage detecting coil does not vary. For this reason, the voltage accuracy at the secondary output is improved.
In addition to the above configuration, assuming that the voltage drop value at the secondary output which is generated by a forward voltage of a rectifying element of the rectifying/smoothing circuit is a first drop value and that the voltage drop value of the voltage detecting signal which is generated by a forward voltage of a rectifying element of the detecting signal creating circuit is a second drop value, the ratio of the first drop value to the second drop value is made approximate to the ratio of the number of turns of the secondary coil to the number of turns of the voltage detecting coil.
Namely, the ratio of the forward voltage of the diode to the output voltage from the secondary coil and the ratio of the forward voltage of the diode to the output voltage from the voltage detecting coil take approximate values to each other. Therefore, the ratios of the influence of the forward voltage which varies according to a temperature to the voltages of the rectified outputs are substantially equal to each other. Thus, even when the forward voltage varies, the voltage of the secondary output to the voltage of the voltage detecting signal does not vary.
In addition to the above configuration, this invention is applied to a high voltage generating apparatus for use in a toner system printing device, in which the detecting signal creating circuit is a double voltage rectifying circuit, and is designed so that the ratio of the current value at the secondary output to the capacitance of a smoothing capacitor in the rectifying/smoothing circuit is made approximate to the ratio of a rectified output current value in the detecting signal creating circuit to the capacitance of a smoothing capacitor in the detecting signal creating circuit.
Namely, the ratio of the voltage of the secondary output to the output voltage from the secondary coil is approximate to the ratio of the voltage of the voltage detecting signal to the output voltage from the voltage detecting coil. Therefore, the ratios of the influence of the forward voltage which varies according to a temperature to the secondary output and the voltage detecting signal takes more approximate values to each other. Thus, even when the forward voltage varies, the ratio of the voltage of the secondary output to the voltage of the voltage detecting signal is maintained at the approximate values.